The present invention relates to a method and an apparatus for correcting the hardening deformation of annular steel elements such as raceway rings of rolling bearings.
Annular elements made of steels which can potentially undergo martensite transformation deform during heat treatments, which cause substantial effects on the quality and cost of the final product. Deformation can occur for the following reasons.
(1) Raw materials already have deformation before being hardened. For example, preliminary working such as lathe turning or cold forging may introduce working strain, or heat treatments such as carburization may introduce residual strain; such strains that have already been produced and retained in raw materials are liberated upon heating prior to the cooling stage of hardening, so that the deformation is caused.
(2) Deformations may also develop due to the thermal and transformational strains that occur during the cooling stage of hardening. The amount of deformations increases upon non-uniform heating or cooling. Consider, for example, the stage of vapor film formation in oil quenching; if the vapor film breaks in a certain area, its heat-insulating effect is lost and the area starts to cool earlier than other areas where the vapor film lingers, so that the coexistence of the two areas causes non-uniform cooling, which eventually results in a deformation.
(3) During hardening, a transformational stress causes not only deformation but also residual internal strain due to the non-uniformity of thermal stress mentioned in (2). Particularly in the case where the correction of deformation is performed during the martensite transformation, the internal strain increases because the external force involved in the transformation is applied to the annular elements. The strain transforms and expands in a direction which the retained austenite liberates the strain during the cooling stage of hardening after finishing the correction of deformation, subsequently during a cleaning step and a tempering step. In short, the deforming force to the annular elements is increased.
Among the three reasons mentioned above, (2) is generally held to be the primary cause of hardening deformation; however, the deformations described under (1) and (3) are also significant as factors to the dispersions in the amount of deformation.
Conventionally, the hardening deformations of annular elements of the kind contemplated by the present invention, particularly those which occur on account of reasons (1) and (2), are corrected by the shrinkage that occurs during the cooling stage of hardening, and by the expansion due to the martensite transformation. FIG. 1 shows the dimensional change that occurs during the heating and cooling of a carbon steel through the martensite transformation. In the illustrated case, the steel being heated from an ordinary temperature start expanding progressively from size A. When the temperature reaches the transformation point, the steel shrinks from size B to C, where it becomes austenite. If heating continues to temperature a, the steel expands from size C to D. If the steel in the austenite region is hardened and rapidly cooled, then it shrinks. But, if it passes through the Ms point in the process of cooling, the steel is so transformed from the austenite to martensite as to expand. In short, the steel expands again at size G where the expansion due to the transformation exceeds the shrinkage due to the decreasing temperature. As the temperature further decreasing, the expansion due to the transformation makes progress and the dimensions of the steel keep increasing. When it has been cooled to an ordinary temperature, its dimensions are greater by a size difference H-A than before it was first heated. According to the general method of correcting deformation, the roundness of an annular element is corrected with a mold by utilizing the shrinking or expanding effect of the cooling stage of hardening compared to the mold size indicated in FIG. 1. To correct the inside diameter of the annular element, it is cooled to temperature b above the Ms, whereupon the mold starts to constrain the inside diameter of the annular element and the correction of its diameter is effected as it subsequently shrinks to the Ms. After the passage of the Ms, the annular element starts to expand and when it has been cooled to an ordinary temperature, it becomes larger than the mold and falls away by itself from the mold. On the other hand, the mold starts to constrain the outside diameter of the annular element at temperature c after it passed the Ms and started to expand. The constraint of the outside diameter of the annular element continues to reach a lower temperature. The cooled annular element must be removed forcedly from the mold.
Conventional methods of correcting the hardening deformation of annular steel elements are described in Unexamined Japanese Patent Publication Nos. Hei. 3-44421, Sho. 62-37315 and 58-31369, as well as Examined Japanese Utility Model Publication No. Sho. 55-13405.
However, these conventional methods have serious problems when put to practical use. If a mold is inserted into the bore of an annular element such that its inside diameter is kept being corrected from the start of hardening to the completion thereof, then the production efficiency drops remarkably compared to the case of employing the normal continuous hardening process. In addition, if the workpiece expands due to the martensite transformation, it bites into the mold so that the constraining ring would leave dents which are detrimental to the success of subsequent finishing and other working operations.
Correction could be effected right after the start of hardening and as long as the temperature is still substantial but not higher than the Ms. However, in this method, the dispersions in the dimensions of the annular element before hardening (the dimensions attained by lathe turning or the dimensions after carburization or carbonitriding) may render the corrective force insufficient or, conversely, an extremely high load becomes necessary to achieve the desired correction. To solve these problems, it becomes necessary to perform an additional step of grinding the annular element before hardening but then the production efficiency drops while the production cost increases.
In the two conventional approaches described above, the correction is performed before the martensite transformation and during it. The transformed portion of the workpiece is significantly improved in strength to develop elasticity and requires considerable force to be corrected. Stated more specifically, the portion of the workpiece that has become martensite as a result of the transformation has experienced the elastic deformation by the forced correction whereas the retained austenite undergoes plastic deformation due to transformation that occurs in the direction of corrective stress. This produces a structure involving much strain due to the mixture of elastic and plastic deformations.
Even if a workpiece which has already been formed in an elliptic form is placed on a round mold to perform correction during the martensite transformation, the workpiece cannot be completely corrected but it merely approaches a round shape and, depending on the ellipticity, some of the annular elements before hardening fail to be corrected in a satisfactory amount. Thus, the correction during the martensite transformation is encountered by difficulty in attaining roundness of high precision.
Rolling bearings of a type that is to be ground after heat treatments suffer from a dual problem in that not only the working efficiency is reduced by the deformation due to the heat treatments but it also varies (disperses) due to the change in the amount of grinding allowance. Stated more specifically, even if the amount of deformation that occurs in an annular element is reduced by performing the correction of deformations making use of the shrinkage that occurs before the martensite transformation and the expansion that occurs thereafter, however, the allowance for the grinding step which is performed subsequent to hardening cannot be significantly reduced if the absolute dimensions of the corrected annular element are variable. For example, in the case where bearings of the same designations are ground in large quantities, even if the amount of deformation is reduced, any dispersions in the absolute dimensions of the corrected annular elements make it impossible to achieve a significant reduction in the grinding allowance.
The dispersions in the absolute dimensions of the annular elements depend in most cases on the precision of lathe turning in the preceding step. In order to achieve dimensional uniformity, grinding may also be performed in the preceding step as already mentioned or, alternatively, cold roll forming is performed as proposed in Unexamined Japanese Patent Publication No. Hei. 6-83872. As already mentioned, the first case is disadvantageous in terms of production efficiency and cost. In the second case which performs strong cold working such as cold roll forming, so much strain is retained in the raw material that dimensional changes or extensive deformation may potentially occur in the subsequent steps of heat treatments.